1. Field of the Invention
The present invention relates to a method and apparatus for preparing a specimen observed with a transmission electron microscope or other similar instrument.
2. Description of Related Art
As a method of preparing a thin-film specimen observed with a transmission electron microscope (TEM), a method described, for example, in Japanese Patent No. 3,263,920, has been heretofore known. In this known method, a shielding material is placed over a specimen. An ion beam is directed from above the shielding material at both shielding material and specimen.
The specimen portions not shielded with the shielding material are ion-etched. At this time, the shielding material is moved in two steps over the specimen and etched to finish the thin-film specimen.
The above-cited Japanese patent has been filed by the present applicant. Last year, the present applicant filed a patent application for a novel specimen preparation method for obtaining thin-film specimens with higher reliability (Japanese Patent Application No. 2004-283802). In this method, as shown in FIG. 1A, a belt-like shielding material is placed over a specimen so as to stand almost uprightly. Ion beams are directed at the shielding material and specimen from leftward and rightward above the shielding material. The specimen is ion-etched to form a through-hole h around the center of the specimen. A peripheral portion A around the through-hole h is a thin film. The thin film A has a thickness adapted for TEM observation. FIG. 1B is a view of the specimen of FIG. 1A, taken from a side surface B.
In the method shown in FIG. 1A, when the through-hole h is formed in the specimen, i.e., when the thin film A having a thickness adapted for TEM observation is completed, the ion-beam irradiation of the specimen must be stopped, for the following reason. If the ion beam is still directed at the specimen even after the through-hole h has been formed in the specimen, the peripheral portion A is etched further and rounded as shown in FIG. 2. As a result, the thickness of the peripheral portion A increases to such a level that TEM observation can no longer be performed.
In this way, in the method shown in FIG. 1A, it is quite important to stop the ion-beam irradiation immediately after formation of the through-hole h in the specimen. We have tried to develop apparatus for automatically stopping the ion-beam irradiation. During the developmental stage, various well-known techniques were attempted to stop the ion-beam irradiation at good timing with unsuccessful results. For example, in one attempt to automatically stop the ion-beam irradiation, light was directed at a specimen, and light transmitted through the specimen was detected. When the intensity of the transmitted light exceeded a threshold value, the irradiation was stopped. However, appropriate timing of stoppage was not achieved. It was not possible to prepare thin-film specimens adapted for TEM observation reliably.